In modern integrated circuits, voltage regulators provide stable voltage references for on-chip blocks such as digital, analog, and RF. An ideal regulator inputs an unregulated voltage from a voltage source, and provides a constant output voltage substantially free of noise or spurs. A typical regulator uses some type of feedback mechanism to monitor and remove variations in the output voltage.
One figure of merit for a regulator is the power supply noise rejection, or PSNR, defined as the ratio of noise appearing on the input voltage to noise appearing on the output voltage. In conventional closed loop regulation, the PSNR is inversely proportional to the loop bandwidth (LBW) of the feedback mechanism. In such designs, power supply noise lying in frequencies beyond the LBW may be hard to remove. On the other hand, a regulator with a wide LBW may consume a great deal of current.
Another figure of merit for a regulator is the dropout voltage. The dropout voltage is the minimum voltage across the regulator required to maintain the output voltage at the correct level. The lower the dropout voltage, the less supply voltage is required, and the less power is dissipated internally within the regulator.
What is needed is a voltage regulator design that provides good PSNR over a wide bandwidth, along with a low dropout voltage.